Method of modulation



Oct. 20, 1936. RQDER I 2,058,262

METHOD OF MODULATION Original Filed Oct. 3. 1930 I 3% J 1 6k INVENTOR HH N s R 0 ER g80 BY 7.- WU

AF z' zw 22] fir-Mm e, ATTORNEY Patented Oct. 20, 1936 V UNITED STATESENT OFFICE METHOD OF MODULATION Hans Roder, Scotia, N. Y., assignor toTelefunken Gesellschaft fiir DrahtloseTelegrap'hie m. b. H., Berlin,Germany, a corporation ofG'e'rmany Application October 3, 1930, serialNO. 486,134. Renewed July 28, 1933. In Germany October 10 Claims.

However, in practice this curve usually departs from a straight linerather appreciably even when the modulation is only around 50% and ismore marked as the modulation increases. Thus, if

one confines oneself to this range, the energy which is radiated isutilized rather poorly, a condition quite distinct from telegraphictransmitter stations which are operable at 100%.

Now, according to this invention modulation up to 100% is to be madefeasible also for telephony transmitter or broadcast stations without anincidental impairment in quality of the transmission being caused, bythat the output potential of the microphone amplifier is so distorted ina suitable manner that as a result such distortions as are due 20 to themodulation curve will be compensated.

Now, for such artificial distortion intended to be used forcompensation, there may be employed the lower knee of the characteristiccurve of a triode tube. Several circuits arranged to practice this 25idea are shown by way of example in the several figures in the drawing.In these circuits of the several figures like reference charactersindicate like parts. I

Figure 1 shows a circuit in which modulating 30 potentials are appliedby way of a modulating tube and circuit including the novel features ofthe present invention to the input circuit of an oscillation repeatertube;

Figures 2 and 3 are curves illustrating the man- 35 ner in whichdistortion is corrected by the use of the novel features of thisinvention;

Figures 4, 5 and '7 show modifications-of the arrangement of Figure 1.The circuit of Figure 4 is especially applicable to modulation schemesof the 40 Heising type;

Figure 6 is a curve illustrating the operation of the modulating circuitillustrated in Figure 7 while,

Figures 8 and 9 are curves showing the relation 5 between currents incertain portions of the circuits of the modulating systems. 7

In describing the invention reference will first be made to Figure 1. InFigure 1 the carrier frequency repeater 2 has an alternating current 50source connected therewith through a transformer "therein. Themodulating potential e1 "is applied "6; is negligible and egm equalsei'.

to the grid of modulatingsignal amplifier tube I in series with anegative gridelectrom'otive force Eg. Iii-parallel relation to ca is avoltage divider -I. Int-he plate circuit of tiibe Iis an electromotiveforce Ea in series with a resistanceea; The 5 grid potential e'gm isobtained at the terminals ab and-KY and is applied to-the modulator tubeII. In the presence of negative or low positive values of-e1 (as gridbiasing potential of I) 'e m==e'1 and is-directly proportional to e1 (inreversed polarity) That is; the polarity of the modulating potentialsapplied to the input electrodes of the "tube II'will be reversed withrespect to the potential applied to the input electrodes of tube I. Thepotential cm is the sum of e1 plus any drop in or resulting from currentflowing in the output ciredit of tube I. When e1 is small the dropthrough As the positive values of ei" grow, there becomes added to (5'1a potential drop occurring across the plate resistance er of tube I.This potential drop increases at more than the linear rate with'respectto the applied potential e 1 and the result is that egm increases nowmore markedly than in direct proportion to e1. This increased potentialat' egm compensates for the normal distortion applied to the modulatingsignal by the modulator tube.

The manner in which this distortion applied to the modulating potentialin the anode of the modulation amplifier I corrects the distortioninherent in modulating systems known hereinbefore will be betterunderstood by reference to Figures 8 and 9 of the drawing.

In Figure 8 the amplitude of the modulated output of a repeater, as, forexample, repeater 2, is plotted as't-he ordinates against the audiofrequency input, which may be applied without distortion to theinputelectrodesoftube II, the latter being plotted-along the abscissae axiaAs will be seen, the relation between the appliedlpotentials 40 V #andvthe "modulations \onthe carrier is not linear.

In Figure 9 the ordinates are again the iamplitudegof; the modulatedoutput of tube 2, but here it is assumed that the audio frequency inputis applied at ei so that they are distorted in theamplifier 1 beforebe-i-ngapplied to the input electrodes of the tube II. This results inalinear relation between the applied modulating potentials and theamplitude of the modulated carrierin the output'electrodes of tube 2.

Figure 2 shows two curves Ja=f(egm) and. Ja 7('er).. [Ja represents theamplitude of the "modulated oscillations appearing in the output ofthethermionic tube II. :91 represents signaling voltages with which theradio frequency "'05- 5 and distorting means of Figure 1.

cillations are to be modulated. The curve Ja=f(l) (function of e1)represents the desired relation between the potential e1 applied to theinput circuit of amplifier I plotted against the amplitude of theoscillations appearing in the output of tube 2. This relation would notbe obtained in usual practice due to distortion in the modulationaction. Applicants novel distorting means may be used to correct thedistortion which inherently results as described above. The lineJa=f(egm) of Fig. 2 represents the voltage egm resulting from thedistortion of the voltage appearing in the output circuit of I whenvoltage e1 is applied to the input, plotted against the amplitude of theoscillations appearing in the output circuit of tube 2. This relation isas shown non-linear. The non-linear relation is of a sense such as tooppose the distortion accomplished during modulation and is sufiicientto compensate such distortion to the desired degree. These curves havebeen plotted on the basis of experimental data. It will be seen thatJa=f(1) is practically rectilinear. The shape of this curve is governeddirectly by the proper choice of the plate resistance. The voltagedivider, the grid biasing potential, and the plate resistance of tube Iserve to adapt this tube to the given curve Ja=f(e m).

The tube I with appropriate associated network such as shown in Figure1, can be used for elimination of distortion in all cases where theIaegm characteristic shows appreciable unilateral curvature for theupper range of Ia (Ir-antenna current) values. A curve of this type isillustrated in Figure 3. Modulation characteristics of the type shown inFigure 3 are encountered if egm is applied to the grid of a driver tubewhich works into a Class B or Class C power output stage. The same typeof Ia-gm characteristic is also encountered when using the modulationmethod which involves working on or varying the grid direct current inthe modulated stage.

Figure 4 illustrates a modified form of applicants novel modulationfrequency amplifying In this arrangement, as will be seen, the cathodeof the tube I is grounded through the distorting resistance er. In otherrespects this arrangement is similar to the arrangement of Figure 1. Theelements shown in Figure 4 may be substituted for the correspondingelements in Figure 1. This may be accomplished by merely breaking thecircuits of Figure 1 at the point KY and connecting the circuit ofFigure 4 to the input of tube II. The circuit arrangement of Figure 4 isespecially applicable to modulating schemes in which Heising modulationis used and when so applied the outputof tube II is connected to theplate circuit of tube 2 in a well known manner rather than to the gridcircuit as in Figure 1.

In Figure 5 has been shown a further modification of the arrangement ofFigure 1. In this modification the distorting resistance er is connectedbetween the cathode of tube I and a point on the resistance 1. Thismodulation frequency amplifying and distorting means may be connectedwith the input circuit of any direct current modulated oscillationsrelay or generator by merely connecting the circuit of Figure 5 to theinput circuit at the points X and Y. The arrangement of Figure 5 isparticularly adaptable to use in circuits where the grid bias of themodulated tube II is controlled in accordance with the correctedmodulating potentials.

When dealing with a characteristic curve having a knee at both ends, asshown in Figure 6, a circuit arrangement including two tubes, connectedas shown in Figure '7, may be used. As will be seen by inspection ofthis arrangement, the distorting resistance er is connected in the leadconnecting the cathodes of tubes I and I so that the distortingresistance is common to circuits of both tubes. This modification andoperation is otherwise similar to the operation of the modulationfrequency amplifying and distorting means described hereinbefore and adetailed description of said operation is thought unnecessary. It willbe noted, however, that this unit may be connected to any modulatingscheme by merely connecting the points X and Y into the modulatingcircuit as described hereinbefore.

Having thus described my invention, I claim:

1. In a signalling system, a carrier frequency thermionic relay, athermionic modulator having its anode to cathode impedance connectedbetween the input electrodes of said relay, a modulation frequencyamplifier, a resistance and a source of potential connected in seriesbetween the input electrodes of said amplifier, means for applying amodulating potential across said resistance, a low impedance connectionbetween the input circuit of said relay and said resistance, aresistance and a source of potential connected directly between theoutput electrodes of said amplifier, and a connection between oneterminal of said last named resistance and the control electrode of saidmodulator tube.

2. In a signalling system, a thermionic relay, a source of carrierfrequency energy, means for connecting the input circuit of said relaywith said source, a modulator tube having its anode to cathode impedanceconnected in series with the input electrodes of said relay, a capacityin parallel with said impedance, a source of modulating signals, amodulating signal amplifier, a resistance connected with said source ofmodulating signals, means for connecting said resistance between theinput electrodes of said modulating signal amplifier, a direct currentconnection between a point on said resistance and the control electrodeof said relay, a resistance and a potential source connected directlybetween the output electrodes of said amplifier, and a connectionbetween said last named resistance and the control electrode of saidmodulator tube.

3. Means for modulating high frequency oscillations and for preventingdistortion of the modulated signal which normally takes place when ahigh degree of modulation is required including, a thermionic repeaterhaving a control electrode, a cathode and an input circuit connectedtherebetween and energized by wave energy of carrier frequency, amodulator tube connected with the input circuit of said repeater, asource of modulating frequency, a modulating frequency amplifier, aresistance connecting said modulating frequency source to the inputelectrodes of said amplifier, a connection between a point on saidresistance and the control electrode of said repeater, a connectionbetween the output circuit of said amplifier and the control grid ofsaid modulator tube, and means for distorting the voice frequenciesrepeated in said amplifier including a resistance and a source ofpotential in the output circuit of said amplifier.

4. In a system for producing an alternating current of an amplitudevarying in accordance with a controlling undulatory potential, a sourceof undulatory control potentials, an alternating current repeater tubeon which alternating current may be impressed, means coupling saidsource and said current repeater tube together to modulate one currentby the other current whereby the amplitude of the said alternatingcurrent energy in said repeater varies in accordance with the amplitudeof the undulatory control potentials, and means interposed between saidundulatory control potential source and said repeater, for altering thewave form of said control potentials as a function of the amplitudethereof whereby the amplitude of said alternating current energy in saidrepeater varies in substantially linear relation with respect to theamplitude of said controlling potential despite distortion whichnormally takes place in said repeater during such operation.

5. In apparatus for producing linear modulation of oscillations, anoscillation repeater comprising an electron discharge device havingwithin an hermetically sealed containeran electron emitting cathode anda control electrode, means for applying oscillations to be modulated to:said device, a source of controlling potentials of varying amplitude,and a non-linear circuit, the amplitude of the potentials at the outputof which vary non-linearly in accordance with the input potentials,coupling said controlling potential source to the control electrode ofsaid oscillation repeater whereby a characteristic of the oscillationsrepeated by said oscillation repeater varies linearly in response to thevariations in potential of the controlling potentials.

6. In apparatus of the character described, a carrier frequency waverepeater comprising, an electron discharge device having with anhermetically sealed container a cathode, and a control electrode, acircuit for applying carrier frequency waves to said control electrode,a source of modulating potentials, a thermionic amplifier having itsinput electrodes connected with said source of modulating potentials,and a circuit including a resistance, the. drop of potential throughwhich is non-linear with respect to the amplitude of the modulatingpotentials, coupling the output electrodes of said amplifier to thecontrol electrode of said carrier wave repeater, whereby theoscillations produced by said oscillator vary linearly in response tothe amplitude of the controlling potentials.

7. In a system for producing linear modulation of a high degree in acarrier wave, a thermionic tube having electrodes including an anode, acontrol grid and a cathode, circuits for applying the carrier wave to bemodulated between the control grid and cathode of said tube, an outputcircuit connected between the anode and cathode of said tube, amodulator tube having an anode, a cathode and a control grid, a circuitconnecting the anode to cathode impedance of said modulator tube betweentwo electrodes of said first named tube, a source of modulatingpotential, a modulation frequency amplifier tube having anode, cathodeand control grid, a circuit connected between said source of modulatingpotentials and the control grid and cathode of said modulation frequencyamplifier tube, an impedance in a circuit between the anode and cathodeof said modulation frequency amplifier tube, and a connection between apoint on said last named circuit and the control grid of said modulatortube and between the cathode of said modulator tube, and a point on thecircuit between said source of modulating potentials and the controlgrid and cathode of said modulation frequency amplifier tube.

8. In a signaling system, a thermionic tube having an anode, a cathodeand a control grid, a circuit to be energized by carrier waves to bemodulated coupled to the control grid and cathode of said tube, analternating current circuit connected to the anode and cathode of saidtube, a modulator tube having an anode, a cathode and a control grid, acircuit coupling the anode and cathode of said modulator tube to thecathode and one of the other electrodes of said thermionic tube, asource of modulating potentials, a first circuit connecting said sourceof modulating potentials to the control grid and cathode of saidmodulator tube for applying thereto modulating potentials, a secondcircuit, having a portion in common with said first circuit, connectingsaid source of modulating potentials to the control grid and cathode ofsaid modulator tube to apply thereto a supplemental modulatingpotential, and means in said second circuit for modifying the wave formof the supplemental modulating potential as a function of the amplitudeof the modulating potentials from said source.

9. A signaling system as recited in claim 8 in which said second circuitcomprises an amplifier tube having an anode, a cathode and a controlgrid, said tube having its control grid and cathode connected to saidsource of modulating potentials and a resistance connecting its anode toits cathode and a connection between its anode and the control grid ofsaid modulator tube.

10. In a signaling system, a thermionic tube having an anode, a cathodeand a control grid, a circuit to be energized by carrier waves to bemodulated coupled to the control grid and cathode of said tube, analternating current circuit coupled to the anode and cathode of saidtube, a modulator tube having an anode, a cathode and a control grid, acircuit coupling the anode and cathode of said modulator tube to thecathode and control grid of said thermionic tube, a source of modulatingpotentials, a first impedance connected therewith, a first circuitconnecting a portion of said first impedance to the control grid andcathode of said modulator tube, a second impedance in said firstcircuit, a thermionic amplifier tube having an anode, a cathode and acontrol grid, a circuit connecting the anode and cathode of saidthermionic amplifier tube to said second impedance, and a circuitconnecting the control grid and cathode of said amplifier tube to saidfirst named impedance.

HANS RODER.

